Electroless nickel plating

ABSTRACT

HIGH STABILITY, AUTOCATALYTIC ELECTROLESS NICKEL PLATING BATH COMPRISING AN AQUEOUS SOLUTION CONTAINING ABOUT 0.08-016 MOLE/LITER NICKEL IONS, ABOUT 0.19-0.38 MOLE/ LITER HYPOPHOSPHITE IONS, AND ESSENTIALLY ABOUT 0.35-3.68 MOLE/LITER AMMONIUM IONS, ABOUT 0.0.-1.07 MOLE/LITER ACETATE ION AND ABOUT 0.007-0.14 MOLE/LITER CITRATE IONS, THE SOLUTION HAVING A PH IN THE RANGE OF ABOUT 7.0 TO ABOUT 9.5 THE AMMONIUM IONS COMPLEX PALLADOUS IONS INTRODUCED INTO THE PLATING BATH BY &#34;DRAG OUT&#34; FROM THE ACTIVATOR SOLUTION TO FORM A SOLUBLE AMMONIUM-PALLADIUM COMPLEX, WHICH INHIBITS REDUCTION OF PALLADOUS ION TO ZERO VALENT CATALYTIC PALLADIUM BY THE HYPOPHOSPHITE OF THE BATH. BY THE REMOVAL OF POTENTIAL CATALYST SITES FROM THE BATH OR BY RENDERING THE POTENTIAL SITES RELATIVELY CATALYTICALLY INACTIVE, RANDOM DEPOSITION OF THE NICKEL AND PREMATURE LOSS OF THE BATH IS AVOIDED.

Unitcd States Patent US. Cl. 117-13011. 2 Claims ABSTRACT OF THE DISCLOSURE High stability, autocatalytic electroless nickel plating bath comprising an aqueous solution containing about 0.08-0.16 mole/liter nickel ions, about 0.19-0.38 mole/ liter hypophosphite ions, and essentially about 0.35-3.68 mole/liter ammonium ions, about 0.09-1.07 mole/liter acetate ions and about 0.0070.14 mole/liter citrate ions, the solution having a pH in the range of about 7.0 to about 9.5 The ammonium ions complex palladous ions introduced into the plating bath by drag out from the activator solution to form a soluble ammonium-palladium complex, which inhibits reduction of palladous ions to zero valent catalytic palladium by the hypophosphite of the bath. By the removal of potential catalyst sites from the bath or by rendering the potential sites relatively catalytically inactive, random deposition of the nickel and premature loss of the bath is avoided.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to electroless nickel plating and more particularly to new and improved autocatalytic high stability chemical reduction nickel plating baths.

(2) Description of the prior art Chemical reduction nickel plating baths heretofore have lacked a high degree of stability and have suffered from a random deposition of metallic nickel and a premature loss of the bath. The problem has resided in drag out of palladous ions, i.e. Pd++ ions, and also metallic palladium particles from the activator solution and their introduction into the chemical reduction nickel plating bath. This drag out and introduction of the palladous ions and metallic palladium particles into the chemical reduction nickel plating bath has been due to the entry of activator solution containing the ionic palladium and/or metallic palladium particles into cracks, holes and/ or fissures frequently present in the plastisol insulation coating of the plating racks after the racks have been used over a considerable time, which racks hold the plastic or other non-conductive articles being electrolessly metal plated during the plating cycle. These cracks and fissures may also be present in the plastisol coating of new plating racks. The drag out of palladous ion-containing activator solution from the activator bath and the introduction of the palladous ions into the nickel plating bath will also occur upon the omission or improper carrying out of the rinsing of the articles after their activating and prior to their immersion into the chemical reduction nickel plating bath. The palladous ions, after entry into the electroless nickel plating bath, are reduced by hypophosphite ions in the bath to form small particles of catalytic palladium metal suspended in the bath. The metallic palladium particles serve as catalytic sites in the bath and catalyze the reduction of the nickel ions therein resulting in unwanted random deposition of metallic nickel, and the premeature loss of the bath.

The presence of palladous ions in the cracks, holes and/ or fissures frequently present in the vinyl plastisol coating 3,597,256 Patented Aug. 3, 1971 of the plating racks also results in the unwanted deposition of metallic nickel on the vinyl plastisol coated racks, by reason of palladium metal catalyst sites being present on the racks due to the ionic palladium being reduced to the zero valent state by the hypophosphite in the plating bath. The electroless deposition of the nickel on the vinyl coated racks is disadvantageous because necessitating a time-consuming and tedious re-racking of the electrolessly nickel plated articles on clean plating racks, prior to the electroplating.

SUMMARY OF THE INVENTION We have found in accordance with the present invention that by utilizing ammonium ions, acetate ions and citrate ions in proportions within certain essential proportion ranges hereafter specified in an electroless nickel plating bath also containing hypophosphite ions and nickel ions, high stability plating baths are provided and which have a considerably reduced tendency to electrolessly plate nickel on the vinyl plastisol coated plating rack. Further, the electroless nickel plating baths are characterized by being low temperature plating baths and effectively plating nickel at room as well as at elevated temperatures. The ammonium ions are utilized in the baths herein in amount of about 0.35-3.68 mole/ liter, the acetate ions in amount of about 0.09-1.07 mole/liter, and the citrate ions in amount of about 0.007-0.14 mole/ liter. The nickel and hypophosphite ions are present therein preferably in amount of about 0.08-0.16 mole/liter of the nickel ions and about 0.19-0.38 mole/liter of the hypophosphite ions, and usually in a ratio Within the molar ratio range of 122-10 of nickel ions to hypophosphite ions respectively.

The ammonium ions complex the palladous ions, i.e. Pd+ ions, introduced into the electroless nickel bath by drag out from the activator solution, to form an ammonium-palladium complex. The formation of the ammonium-palladium complex inhibits or retards reduction of the palladous ions to zero valence catalytic palladium metal by the hypophosphite ions of the electroless bath. Consequently, by the removal of potential catalyst sites from the bath and from the plating rack, random deposition of nickel and premature loss of the bath is avoided. Further, nickel deposition on the vinyl-coated plating rack is considerably lessened. The 0.35-3.68 mole/liter of ammonium ions is an essential range of such ions inasmuch as with amounts of ammonium ions much above 3.68 mole/liter, there tends to occur skip plating when nickel is present in the aforementioned molar range thereof. Amounts of ammonium ions much below 0.35 mole/liter should be avoided to avoid the presence of insuflicient ammonium ions to complex the dragged in palladous ions. The 0.09-1.07 mole/liter range of acetate ions is an essential range of this constituent for the reason that with amounts of acetate ions much above 1.07 mole/liter in the electroless bath, there is the likelihood a loss of stability of the bath will occur and a bath of appreciably shorter life will result. With amounts of acetate ions much below 0.09 mole/liter, the electroless nickel plating bath only has a useful life of about 4 days due to a too tightly bound nickel in a Ni-citrate ion complex in the bath. The acetate ion forms a less tight complex with a portion of the nickel ions which enables nickel to be plated for a period considerably longer than four days. The 0007-014 mole/liter of citrate ions in the bath is an essential range for the citrate ions inasmuch as at quantities of citrate ions much above 0.14 mole/liter, the ionic nickel is too tightly complexed in a nickel-citrate complex with the result that high temperatures of about F.-200 F. are required for the plating bath during the electroless plating. The requirement of such high plating bath temperatures is disadvanta- 3 geous when heat sensitive surfaces or substrates are being plated which are detrimentally affected by these temperatures, for example heat-sensitive plastic surfaces, e.g., acrylonitrile-butadienestyrene resin surfaces having a heat distortion point of about 180 F. With an amount of citrate ions much below 0.007 mole/liter, the desired good stability of the bath is lost and the plating bath is one of inherently shorter life.

PREFERRED EMBODIMENTS OF THE INVENTION The high stability electroless nickel plating bath of this invention preferably comprises an aqueous solution preferably containing about 0.08-0.16 mole/ liter nickel ions, about 0.19-0.38 mole/liter hypophosphite ions, about 0.5-2.0 mole/liter ammonium ions, about 0.09- mole/ liter acetate ions, and about 0007-014 mole/ liter citrate ions. These preferred baths have a pH in the range of about 7.0 to about 9.5. The nickel ions and hypophosphite ions are preferably present in the bath in a ratio within the molar ratio range of 1:2-4 of nickel ions to hypophosphite ions respectively.

In the electroless nickel plating bath of this invention, the nickel ions are supplied by any suitable source of nickel ions such as, for example, one or more watersoluble nickel salts, e.g., nickel chloride, nickel sulfate, nickel acetate, nickel ammonium sulfate and nickel hypophosphite. Ammonium ions are supplied to the bath by any suitable source of ammonium ions such as, for example, a suitable ammonium compound exemplified by ammonium hydroxide ammonium chloride, ammonium acetate, ammonium sulfate and ammonium hypophosphite. At the outset or beginning of use of the nickel plating baths herein, the ammonium ions are usually supplied in the bath by ammonium hydroxide, and the concentration of ammonium ions is preferably maintained within the essential proportion range previously specified herein by the addition to the bath of an ammonium salt, e.g., ammonium chloride. Citrate ions are supplied to the bath by any suitable source of such ions such as, for example, a water-soluble salt of citric acid as exemplified by an alkali metal citrate, e.g., sodium or potassium citrate, or citric acid. Acetate ions are supplied to the bath by any suitable source of acetate ions, preferably acetic acid and sodium acetate trihydrate. Anhydrous sodium acetate and ammonium acetate are not preferred as the source of acetate ions in the nickel plating bath of this invention as use of such acetate tends to result in undesirable skip plating of the article surface. Skip plating is that plating where a portion or portions, which may be microscopic in size or of relatively large size visible to the naked eye, of the plastic article surface are not metal plated by the chemical reduction metal plating bath and the remaining portion or portions of such article surface are metal plated by the plating bath. Hypophosphite ions are supplied to the bath by any suitable source of such ions such as, for example, sodium or potassium hypophosphite, ammonium hypophosphite or nickel hypophosphite.

One preferred high stability electroless nickel plating bath of this invention comprises an aqueous solution having dissolved therein about -45 grams/liter of nickel sulfate, about 10-50 grams/liter of sodium hypophosphite, about 10-60 grams/liter of ammonium chloride, about 2-40 grams/liter of sodium citrate, about 5-63 grams/liter of glacial acetic acid, and about 6-144 grams per liter of ammonium hydroxide. Such bath has a pH in the range of about 7.0 to about 9.5.

The pH of the alkaline electroless nickel baths herein are maintained Within the pH range aforementioned by addition of ammonium hydroxide.

The surfaces capable of being nickel plated with the electroless nickel plating baths herein are both electrically non-conductive or nonmetaliic surfaces and metallic surfaces. Nonmetallic surfaces platable herein are exemplified by organic plastic surfaces, e.g., surfaces of acrylonitrile-butadiene-styrene resins, epoxy resins, polypropylene, polysulfone and polystyrene. The object or article may be formed entirely of plastic, or partially of plastic with the plastic forming a surface or surfaces and being secured or bonded to another material. The metallic surfaces are exemplified by surfaces of ferrous metal, e.g., steel, nickel, cobalt or palladium.

The metallic surfaces prior to being plated with nickel by this invention, if not already in catalytically activated condition for the plating, are subjected to a conventional cleaning and pickling. The metal substrates are then catalysts for the deposition of nickel from the plating baths herein by the redox reaction.

Prior to electrolessly plating, the electrically non-conductive surfaces such as the plastic surfaces, if not already clean, are cleaned by immersion in a conventional nonsilicated mild alkaline cleaner solution. The plastic surface or surfaces intended to be electrolessly metal plated are then converted from a hydrophobic state to a hydrophilic state wherein the surfaces are readily receptive to the aqueous solutions of the chemical reduction metal plating process. The conversion of the hydrophobic plastic surfaces to hydrophilic surfaces is preferably effected by contacting the hydrophobic surface with, usually by immersing such surface in, a chromic acidand sulfuric acidcontaining aqueous conditioning or etching solution at a solution temperature generally in the range of l10-l90 F. with the particular temperature depending on the particular polymer plated. Such conditioning solution may also contain phosphate ions. A typical conditioning solution contains, by Weight, 27.5% of 66 B. H 27.5% of CrO and 45% of H 0. The conversion of the hydrophobic plastic surfaces to hydrophilic surfaces can also be effected mechanically by roughening or deglazing the plastic surface, for instance by sanding or abrading the hydrophobic plastic surface. When such a mechanical conversion is employed, the prior chemical cleaning of dirty plastic surfaces may be omitted as the mechanical roughening or deglazing itself effects a cleaning of the plastic surface.

The conditioned or etched plastic surfaces are then thoroughly rinsed with Water prior to sensitizing.

The plastic articles are then sensitized by contacting the plastic surfaces or surface with, usually by immersing the surface in, a conventional sensitizer aqueous solution containing stannous chloride, hydrochloric acid and water. The contact time with the sensitizer solution is typically one minute, and the sensitizer solution is usually at room temperature during the contacting of the plastic surfaces therewith. A typical sensitizer solution for use herein is the following:

SnCl 10 g. HCl40 ml. 11 0-1000 ml.

The sensitized articles are then removed from the solution and Water rinsed.

The sensitized plastic surfaces of the articles are then activated by contacting the surfaces with, usually by immersing the sensitized surfaces in, an activator solution containing a noble metal salt, preferably palladous chloride, and water, at typically room temperature of the solution and typically for 1 minute. A typical activator solution for use herein is the following:

PdC1 1 g. HCl-10 ml. H O-l gal.

The activated plastic article is then separated from the activator solution and rinsed with Water.

Although it is not preferred, the sensitizing and activating steps may be reversed in sequence with the sensitizing following the contacting of the plastic surface with the activator solution.

The activating may also be performed from a single solution containing both the activator and sensitizer.

The activated plastic surfaces of the article are then electrolessly nickel plated by contact with, usually by immersion in, the chemical reduction nickel plating solution of this invention at typically room temperature of the solution. The electroless nickel plating is carried out for a time sufiicient to render the activated surface or surfaces electrically conductive or until a nickel deposit of the desired thickness is formed. The thus plated article or articles are then removed from the electroless plating solution followed by water rinsing the article.

The nickel-plated surfaces can then be electroplated in conventional manner with for example, nickel, copper, or nickel and copper to increase the thickness of the metal layer. A final or finish metal layer, for example of chrome, can thereafter be electroplated also in conventional manner on the first-mentioned electroplate layer.

The following examples of high stability, autocatalytic electroless nickel aqueous plating bath solutions further illustrate the invention but are not restrictive thereof.

EXAMPLE 1 G./l. NiSO -6H O 43 (0.16 M Ni++) NH Cl (0.56 M NHU) N3-3C5H507'2H20 5 M C6H5O'1E) CH COOH (glacial) 9.5 (0.16 M CH COO NH-HzPOz'HzO M NH OH (26 B) (0.79 M NH pH 8.2-8.7 Operating temperatureroom temperature150 F.

EXAMPLE 2 G./l. NiSO -6H O 43.0 (0.16 M Ni++) NaH PO -H o 40.0 (0.38 M H2PO2 NH Cl 30.0 (0.56 M HNJ) NH OH (26 B) 27.0 (0.48 M NH Na C H O -2H O 10.0 (0.03 M C H O CH COOH (glacial) 21.0 (0.36 M CH COO) pH about 0.3 Operating temperatureroom temperature-150 F.

EXAMPLE 3 G./l. NiSO -6H O 43.0 (0.16 M Ni++) NaH PO -H O 40.0 (0.38 M H2PO2 NH Cl 30.0 (0.56 M NH NH OH (26 B) 18.0 (0.32 M NH Na C H O- -2H O 10.0 (0.03 M C H5Oq CH COOH (glacial) 9.5 (0.16 MCH COO pH about 8.510.? Operating temperatureroom temperature-150 F.

CH COOH (glacial) pH about 8.5103

Operating temperatureroom temperature-150 F.

5.3 (0.09 M CH COO) EXAMPLE 5 G./l. NiSO -6H O 43.0 (0.16 M Ni++) NaH PO -H O 40.0 (0.38 M H2PO2 NH Cl 30.0 (0.56 M NH.;'*) NH OH (26 B) 72.0 (1.27 MNH Na C H O -2H O 20.0 (0.69 M C6H5O7E) CH COOH (glacial) 9.5 (0.16 MCH COO) pH about 9.0;t0.2 Operating temperatureroom temperature150 F.

EXAMPLE 6 G./l. NiSO -6H O 43.0 (0.16 M Ni++) NaH2PO2H2O M H2PO2 NH Cl 30.0 (0.56 M NHJ) NH OH(26 B) 144.0 (2.56 M NH Na-Acetate-3H O 27.5 (0.34 M Acetatr) Na C H O -2H O 20.0 (0.07 M C H 0 pH about :0.2 Operating temperatureroom temperature- F.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

What is claimed is:

1. An autocatalytic high stability electroless nickel plating bath comprising an aqueous solution containing about 0.08-0.16 mole/liter nickel ions, about 0.19-0.38 mole/ liter hypophosphite ions, about 0.35-3.68 mole/liter ammonium ions, about 0.09-1.07 mole/liter acetate ions supplied by an acetate ion source other than anhydrous sodium acetate, and about 0.07-0.14 mole/liter citrate ions, the solution having a pH in the range of about 7.0 to about 9.5, the solution effectively plating nickel on an object catalytic surface at a temperature in the range of room temperature to 150 F.

2. A method for electrolessly nickel plating a catalytic surface of an object, which comprises contacting the catalytic surface with an autocatalytic high stability chemical reduction nickel plating bath comprising an aqueous solution containing about 008-016 mole/ liter nickel ions, about 0.19-0.38 mole/liter hypophosphite ions, about 035-368 mole/liter ammonium ions, about 0.09-1.07 mole/liter acetate ions supplied by an acetate ion source other than anhydrous sodium acetate, and about 0.007- 0.14 mole per liter citrate ions, the solution having a pH in the range of about 7.0 to about 9.5 and being at a temperature in the range of room temperature to 150 F., until a nickel deposit of the desired thickness is formed on said surface.

References Cited UNITED STATES PATENTS 2,865,375 12/1958 Banks et al 117130X 3,060,059 10/1962 Sickles 117130 3,148,072 9/1964 West et a1. 1061 3,178,311 4/1965 Cann 1l7160 3,466,232 9/ 1969 Francis et a1 117--47 ALFRED L. LEAVITT, Primary Examiner E. G. WHITBY, Assistant Examiner US. Cl. X.R.

117-47A, 47H, 71R, 71M, 47R, 138.8E, 138.8UA, R; 106 l; 204-38 @2 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3 597.266 Augus t 3+ l2ll Inventor(s) Gary Leibowitz and Richard L. Mullanev Patent No.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 69, "premeature" should read --premature--. Column 5, line 39, "(0.56 M HN should read --(0.56 M NH line 42, "(0.36 M CH COO')" should read --(0.36 M CH3COO") Column 6, line 36, "0.07-0. 14" should read --0.007-O. l4--.

Signed and sealed this 15th day of February 1972.

(SEAL) At best EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Commissioner of Patents Attesting Officer 

